In general, an internal combustion engine and a transmission are installed in a vehicle such as a car, a truck, and a bus, and a medium circulates through the internal combustion engine and the transmission. Internal combustion engine circulating oil (engine oil) used for lubricating a driven part, driving a movable part, and cooling a heated part when the internal combustion engine is driven, represents the medium that circulates through the internal combustion engine. Further, coolant water is a refrigerant or the medium that suppresses rise of a temperature of the internal combustion engine when the internal combustion engine is driven. On the other hand, transmission circulating oil (mission oil) used for lubricating the driven part, driving the movable part, and cooling the heated part when the transmission changes an output of the internal combustion engine depending on a driving state to transmit the output to a road through a wheel, is a medium that circulates through the transmission.
In general, friction at a low temperature state of the internal combustion engine is preferably reduced when the internal combustion engine is started in order to improve startability. Further, the temperature of the internal combustion engine is preferably raised in a short time from the low temperature state while warming up the internal combustion engine in order to improve warm up ability. Here, the medium circulates through the internal combustion engine and through the transmission at the low temperature state even at the starting and the warming up of the internal combustion engine. Therefore, startability and warm up ability of the internal combustion engine cannot be improved since the medium at the low temperature state circulates therethrough.
In the engine oil and in the mission oil for example, the friction cannot be reduced compared to the friction thereof at a high temperature even if the driven part of the internal combustion engine or the transmission that is connected to the internal combustion engine is lubricated since viscosities of the engine oil and the mission oil increase as the temperatures thereof decrease. Further, a rate at which the temperature of the internal combustion engine rises slows down as the temperature of the coolant water decreases since the coolant water receives heat generated by the internal combustion engine through which the coolant water circulates.
A technique in which an apparent viscosity of the oil is decreased is proposed. For example, Japanese Utility Model Application Laid-Open (JP-U) No. S63-78122 discloses a cooling device that cools the internal combustion engine by circulating the oil through an oil jacket formed on a cylinder block of the internal combustion engine. In the cooling device, a bubble generator is disposed at a predetermined position of the oil jacket to mix fine bubbles into the oil so that the apparent viscosity of the oil is decreased.
In JP-U No. S63-78122, air is injected from one side of an ultrafine fine mesh net and one side of ultrafine small holes drilled and formed on a surface of the bubble generator, to other side thereof to generate ultrafine bubbles. A diameter of the ultrafine bubble is required to be less than or equal to 1 mm. However, the bubbles generated by the small holes and the net can be visually recognized even if the bubbles are said to be small, because for example, the bubble has the diameter of approximately 0.2 mm. Therefore, the bubbles might become larger bubbles by absorbing and combining with each other. Consequently, a pump that sucks and discharges the oil sucks the large bubbles when the oil is circulated, and discharge ability of the pump might decrease. Thus, friction might increase since enough engine oil is not supplied to a section to be lubricated of the driven part of the internal combustion engine.